1. Field of the Invention
The present invention relates to a fuel cell stack having stacked unit fuel cells, each having a membrane electrode assembly in which an anode and a cathode are provided on either side of an electrolyte membrane, and the membrane electrode assembly is placed between metal separators for supporting the membrane electrode assembly.
2. Description of the Related Art
Typically, solid polymer-type fuel cells have a membrane electrode assembly in which an anode and a cathode are provided on either side of a solid polymer electrolyte membrane consisting of a polymer ionic exchange membrane (i.e., cation exchange membrane). The membrane electrode assembly is placed between separators which are provided for supporting the membrane electrode assembly, so as to form a unit fuel cell. Generally, a specific number of unit fuel cells are stacked to obtain a fuel cell stack.
In this kind of fuel cell stack, a fuel gas supplied to the anode, such as a hydrogen gas, is ionized to hydrogen ions on catalytic electrodes, and the hydrogen ions are transferred to the cathode via an electrolyte membrane which is humidified to have an appropriate level of humidity. During this process, electrons are generated and flow to an external circuit, providing DC (direct current) electric energy. An oxidizing gas such as oxygen or air is supplied to the cathode, and the hydrogen gas, electrons, and oxygen gas react at the cathode, thereby generating water.
Generally, in such a fuel cell stack, the separators are made of carbon which has superior electric characteristics (e.g., conductivity) and superior corrosion resistance, and the separators made of carbon can be precisely manufactured. However, the thickness of a separator made of carbon is large, and the strength and productivity of such a separator are inferior. In consideration of these circumstances, metal separators have been proposed, which can be thin and be formed by press working, and high productivity can be realized. A technique related to the metal separators has been known (refer to Japanese Unexamined Patent Application, First Publication No. 2000-21419).
As shown in FIG. 4, in the fuel cell stack employing metal separators, a unit fuel cell 107 is constructed by placing a membrane electrode assembly 104 between a pair of corrugated metal separators 105 and 106, where in the membrane electrode assembly 104, a solid polymer electrolyte membrane 101 is placed between an anode 102 and a cathode 103. Plural unit fuel cells 107 are stacked in a manner such that convex portions 105a and 106a of the adjacent metal separators 105 and 106 contact each other and concave portions 105b and 106b of the adjacent metal separators 105 and 106 face each other.
Here, each space between the concave portions 105b and 106b (which face each other) of the metal separators 105 and 106 is used as a passage 109 through which a cooling medium flows. In addition, each space between the anode 102 and a concave portion 105c which is a reverse side of the convex portion 105a of the metal separator 105 is used as a passage 110 through which a fuel gas flows. Similarly, each space between the cathode 103 and a concave portion 106c which is a reverse side of the convex portion 106a of the metal separator 106 is used as a passage 111 through which an oxidizing gas flows.
However, the metal separators made by press working tend to be deformed and to have an error in shape such as a camber or waviness, and consequently, necessary flatness may not be obtained. In particular, if a load is imposed due to fastening on the fuel cell stack, the unit fuel cells 107 including the metal separators may become cambered or the unit fuel cells 107 may be deformed. Consequently, the pressure imposed on each electrode surface of the anode and cathode adjacent to the deformed portion is not uniform, so that the contact resistance increases and the internal resistance of the fuel cell also increases. Accordingly, required performance of the fuel cell stack cannot be obtained. In addition, it is difficult to maintain required sealing capability between the metal separators. Furthermore, the above-explained errors in shape are cumulative due to a stacked structure, thereby reducing the dimensional accuracy of the fuel cell stack.